1. Field of the Invention
The present invention relates to a solid electrolytic capacitor formed by connecting a capacitor element to a cathode lead frame.
2. Description of the Background Art
A solid electrolytic capacitor having a structure as shown in FIG. 7 has been known as a conventional solid electrolytic capacitor. This solid electrolytic capacitor includes a capacitor element 91, an anode lead frame 93, a cathode lead frame 94, and a molded resin 92. As shown in FIG. 8, capacitor element 91 is constituted of an anode element 911 on which an anode lead member 912 is erected, a dielectric film 913 formed on an outer peripheral surface of anode element 911, a solid electrolyte layer 914 formed on dielectric film 913, and a cathode draw-out layer 915 formed on solid electrolyte layer 914. Anode element 911 is formed of a sintered object of a valve metal (such as tantalum, niobium, titanium, and aluminum).
Anode lead member 912 constituting capacitor element 91 is electrically connected to anode lead frame 93 through resistance welding, laser welding or the like. Cathode draw-out layer 915 of capacitor element 91 is electrically connected to cathode lead frame 94 through a conductive adhesive material 95. Anode lead frame 93 and cathode lead frame 94 are drawn out of molded resin 92 and bent along a side surface and a lower surface of the solid electrolytic capacitor.
In addition, a solid electrolytic capacitor having a structure as shown in FIG. 9 has also been known as another conventional solid electrolytic capacitor (for example, Patent Document 1: Japanese Patent Laying-Open No. 2005-101480). This solid electrolytic capacitor is different from the conventional solid electrolytic capacitor shown in FIG. 7 in that two through holes 941 are provided in cathode lead frame 94. Through holes 941 are filled with conductive adhesive material 95.
In a process for manufacturing the conventional solid electrolytic capacitor shown in FIG. 7, however, capacitor element 91 has been bonded to cathode lead frame 94 by placing capacitor element 91 on cathode lead frame 94, for example, with a conductive adhesive liquid 95L mainly composed of silver being interposed, thereafter performing heating and curing conductive adhesive liquid 95L.
Meanwhile, in order to enhance conductivity, a material mainly composed of copper has been used for cathode lead frame 94, however, such cathode lead frame 94 is greater in coefficient of linear expansion than conductive adhesive material 95 or capacitor element 91 and large shearing stress remains between cathode lead frame 94 and conductive adhesive material 95 during heating followed by cooling. Therefore, a crack may be produced in conductive adhesive material 95 after such electronic devices as a portable telephone or a computer containing the solid electrolytic capacitor have been marketed. Thermo cycle tests for testing change over time revealed low reliability, because ESR was poor due to production of a crack in conductive adhesive material 95.
In addition, since conductive adhesive liquid 95L lies between capacitor element 91 and cathode lead frame 94, a solvent in conductive adhesive liquid 95L is difficult to vaporize and escape to the atmosphere during the heating step and the vaporized solvent may remain as bubbles in cured conductive adhesive material 95. If bubbles are formed in conductive adhesive material 95 as such, bonding strength between cathode lead frame 94 and capacitor element 91 is lowered and equivalent series resistance (ESR) disadvantageously deteriorates.
Moreover, in the conventional solid electrolytic capacitor shown in FIG. 9, conductive adhesive material 95 introduced in hole 941 is exposed at a surface of cathode lead frame 94 and further protrudes into molded resin 92 (see FIG. 1 of Patent Document 1). A distance between a tip end of a protruding portion 95A of conductive adhesive material 95 and the outer periphery of molded resin 92 is thus smaller than a distance between the surface of cathode lead frame 94 and the outer periphery of molded resin 92. If external force is applied to that portion, that portion may be deformed and broken (that is, mechanical strength may become lower). Further, as a degree of protrusion is greater, protruding portion 95A is exposed through molded resin 92, which in turn gives rise to such problems as poor moisture-resistance characteristics or poor appearance. In order to suppress the degree of protrusion to a prescribed level or lower, process management becomes complicated and hence cost is increased.
Furthermore, the vaporized solvent of conductive adhesive material 95 is exhausted to the atmosphere through hole 941, however, a cross-sectional area of hole 941 is small and an amount of exhaust is small, which leads to formation of bubbles in conductive adhesive material 95. If the cross-sectional area of hole 941 is made larger in order to avoid formation of bubbles, the degree of protrusion of conductive adhesive material 95 becomes greater and the aforementioned problem is more likely.